1. Field of the Invention
The present invention relates to an injection molding method of a molded article of a polyamide resin represented by Nylon (under trade name).
2. Description of the Related Art
Ordinarily, a linear polymer having an amide bond as a repeating unit in a molecule is called a polyamide and, among other things, the polyamide mainly comprising an aliphatic chain is widely distributed as a resin under a trade name of xe2x80x98Nylonxe2x80x99 represented by Nylon 66, Nylon 6 and the like. Since a polyamide resin comprises an amide group having a large polarity, the polyamide resin has an excellent resistance to hydrocarbon type solvents such as gasoline, oil and the like. Further, since the polyamide resin has a relatively high heat resistance, it has been used as a favorable material for components of automotive vehicles, components of machinery and the like. Meanwhile, an integrally molding method that comprises the steps of continuously fabricating molded articles by an extrusion molding process and the like which has a high productivity to prepare an already-molded article, setting the thus-prepared already-molded article in a metallic mold and separately injection-molding only a complicated part which can not be molded by extrusion molding to the thus-set already-molded article is effective, since it can save cost and labor required for injection-molding a final molded article as an entire body.
However, even when, for aiming at performing molding by using the polyamide resin by means of the above-described method, a molten polyamide resin is injection-filled against a predetermined integral molding part of the already-molded article comprising the polyamide resin which has already been molded by using extrusion molding or the like, the molten polyamide resin and the already-molded article are not integrally bonded to each other such that they separate from each other as they are cooled. It has ordinarily been known that, even when a molten resin, of a crystalline polymer such as the polyamide resin, which has a same composition as that of the already-molded article is filled against the already-molded article in a metallic die for injection molding, there occurs no integral bonding therebetween. The reason is surmised as described below. It is required for heating the metallic die to hold a predetermined temperature when the already-molded article is set in the metallic die and, then, the molten resin is filled for an addition molding part. On this occasion, since crystallization of the already-molded article in the metallic die is promoted, bonding strength thereof against an amorphous molten resin to be used for the additional molding part is decreased. This is because it is considered that, when the crystalline polymer is heated, partial melting occurs therein at a considerably lower temperature than a melting point thereof and, subsequently, recrystallization occurs therein. Therefore, the already-molded part and the molten resin part become each individually crystallized where upon they are not bonded to each other. Since a crystal structure of the polyamide resin is firm with an involvement of a hydrogen bond between amide bonds, it has been difficult to perform integration.
It has been known that, in the case described above, particularly in a case of a polyethylene resin molded article or the like which is a crystalline polymer similar to the polyamide resin, when, after an acetone type primer is applied to a predetermined integral molding part of the already-molded article, the above-described injection-addition molding operation is performed, the already-molded article and the resultant molded part are bonded to each other. The primer on this occasion denotes an organic solvent to be applied in advance as a pretreatment agent to the predetermined bonding part of the already-molded article and, for this application, the primer mainly comprising acetone has conventionally been distributed in the market. However, when, in the case of the polyamide resin, the present inventor performed such a molding operation by applying the acetone type primer to the polyamide resin, a molded article having a sufficient boding strength was unable to be obtained.
Further, as an integral injection molding method of a polyamide resin molded article having a sufficient bonding strength at a bonded part between the already-molded article and the additional molding part, a resistance welding method has been known. It is a method in which an electrically conductive wire is incorporated into the already-molded article along a periphery of the predetermined integral molding part of the already-molded article and, then, heated by an introduction of electric current to a hot wire whereby the above-described predetermined integral molding part is allowed to be slightly melted to be in an amorphous state and, then, the polyamide molten resin for the additional parts injection-filled in the metallic die such that the polyamide molten resin covers the predetermined integral molding part to perform molding. In a case of this method, there are mentioned problems described below. (1) Working efficiency in performing incorporation of the electrically conductive wire is extremely low whereupon, particularly, incorporation work against a small-sized molded article becomes difficult. (2) Since it takes time to melt the already-molded article comprising the polyamide resin by the hot wire, production efficiency at the time of injection molding is low. (3) Since the electrically conductive wire remains in the polyamide resin after the injection molding has been executed, when the resin molded article is recovered for the purpose of reuse after a product comprising such a resin molded article has been used, it is impossible to reuse the part thereof incorporated with the electrically conductive wire. (4) Since a part of the molten resin corresponding to a volume of the electrically conductive wire is squeezed out to be a burr when the molded article is released from the metallic die whereupon there is a need for a finishing treatment.
An object, therefore, of the present invention is to provide a method in which, when an additional molding part comprising a same resin as that of an already-molded article which comprises a polyamide resin is injection-molded against the already-molded article, they are integrated while securing a sufficient bonding strength therebetween only by an application of a primer.
A first aspect of the present invention is a method in which an already-molded article comprising a polyamide resin is set in a molding die and, then, the polyamide resin that has been heated to be in a molten state is injection-filled in a cavity to which a part of the already-molded article faces thereby integrally molding an additional molding part onto the part of the already-molded article, characterized in that a primer comprising an alcohol solution containing the polyamide resin and a chemical that is capable of dissolving the polyamide resin is applied to a predetermined integral molding part of the already-molded article with the additional molding part and, then, the thus primer-applied already-molded article is set in a molding die.
According to the first aspect of the present invention, at the time of molding the polyamide resin, when a molded article having an intended shape is molded by firstly molding in advance a part having a shape suitable for continuous molding such as extrusion molding, blow molding or the like to prepare an already-molded article and, then, injection-molding a only part having a complicated shape which is incapable of being continuously molded in an additional manner against the thus-prepared already-molded article thereby integrally bonding them, it is possible to integrate an additional molding part while having a sufficient bonding strength by injection molding only by adding such a simple operation as to apply the above-described primer to the predetermined integral molding part of the above-described already-molded article. Therefore, a complicated operation, such as incorporating an electrically conductive wire into the predetermined integral molding part is not necessary. Further, ordinarily, when the additional molding part, is injection-molded against the already-molded article, it is necessary to control a temperature of the molding die such that it is allowed to be higher than an ordinary molding temperature; however, when an integration is performed with an application of the primer, the integration can be performed without depending on the temperature of the molding die. Furthermore, there is no fear of generating a burr whereby productivity of the additional injection molding operation can be enhanced. Still furthermore, when the integrally molded article is melted, an original resin can be recovered without generating wastes whereupon the integrally molded article is suitable for reuse.
The injection molding method of the polyamide resin molded article according to the present invention is characterized in that an alcoholic primer which is different from the conventional acetone type primer containing acetone as a main component is applied. (hereinafter, the primer according to the present invention is referred to as alcoholic primer). The term xe2x80x9calcoholic primerxe2x80x9d used herein is intended to include an alcohol solution of the polyamide resin containing a chemical which dissolves the polyamide resin. As an optimum chemical which dissolves the polyamide resin according to the present invention, mentioned are phenols such as phenol, 1,3-dihydroxybenzene and the like. Further, as solvents, which are capable of being easily evaporated after being applied, for use in the polyamide resin and the chemical which dissolves the polyamide resin, mentioned are alcohols each of which has carbon atoms of from 1 to 6 and has a hydrogen bond in a same manner as in the polyamide resin; on this occasion, the alcohols may be a mixture of different types of alcohols. The polyamide resin may be of any type so long as it is the same as in cases of the already-molded article and the additional molding part.
It is well known in the art that the polyamide resin is a crystalline polymer in which a hydrogen bond can be formed between amide bonds of different polymer chains and, since an intermolecular force acts strongly in a crystalline region thereof, the polyamide resin is not readily dissolved and, therefore, only a minute change is noticed therein against acetone. On the other hand, by applying the alcoholic primer to the predetermined integral molding part of the already-molded article, due to an action of the chemical which dissolves the polyamide resin contained in the alcohol solution, the polyamide resin of the predetermined integral molding part is partially dissolved so as to be in an amorphous state which allows a space between the polymer chains thereof to be enlarged. Then, the polyamide resin contained in the alcoholic primer is intertwined with such polymer chains of the polyamide resin in a disorderly manner to be like a wedge. Although it is considered that the alcoholic solvent is evaporated after being applied, such a wedge made of the polymer chains of the polyamide resin in the alcoholic primer and the chemical which dissolves the polyamide resin are left on a surface of the predetermined integral molding part to maintain an amorphous state. When a molten resin for the additional molding part is injection-filled against the predetermined integral molding part, both polyamide resins which are each in an amorphous state are intertwined with each other in a disorderly manner by being subjected to heat of the molten resin. Namely, when cooled in the molding die, the predetermined integral molding part and the additional molding part are prevented from being each individually recrystallized by the act of the stereo-configuration type wedge of the polymer chains of the polyamide resin contained in the alcoholic primer whereupon the already-molded article and the additional molding part can be integrated while having a sufficient bonding strength therebetween.
A second aspect of the present invention is characterized in that the primer according to the first aspect of the present invention is a solution comprising 1,3-dihydroxybenzene of from 20% by weight to 25% by weight, a polyamide resin of from 10% by weight to 20% by weight and methanol of from 60% by weight to 70% by weight.
When 1,3-dihydroxybenzene which acts to enlarge an intermolecular space of the polyamide resin is over a range of from 20% by weight to 25% by weight, since melting of the predetermined integral molding part of the already-molded article excessively advances whereupon it inappropriately becomes difficult to retain an original shape, while when 1,3-dihydroxybenzene is below the above-described range, a region of an amorphous state is not sufficiently spread therein whereupon a sufficient strength for integration cannot be held. Further, when the polyamide resin is over the range of from 10% by weight to 20% by weight, a viscosity of the alcoholic primer becomes high whereupon a handling property thereof is deteriorated, while, when it is below the above-described range, it becomes difficult to allow the polyamide resin to function as a wedge whereupon a sufficient strength for integration can not be held. Methanol, being small in molecule, is a solvent which can easily approach various types of larger molecules and also a solvent which, among alcohols, exhibits a strong hydrogen bond property to dissolve 1,3-dihydroxybenzene. When the primer solution contains methanol of from 60% by weight to 70% by weight, the handling property thereof is favorably held.
A third aspect of the present invention is characterized in that the polyamide resin according to the first or second aspect of the present invention is an aliphatic nylon resin.
Examples of aliphatic nylon resins include nylon 6, nylon 66, nylon 11, nylon 12, nylon 610, nylon 612, nylon 69 and the like and, further, include mixtures of two types or more of the aliphatic nylon resins or copolymers thereof such as nylon 6/nylon 66, nylon 6/nylon 11, nylon 66/nylon 610 and the like and, still further, include polymer alloys of the above-described aliphatic nylon resins blended with polyolefin type rubber and the like. According to the third aspect of the present invention, since there is a degree of freedom to some extent in the stereo-configuration of a side-chain methylene group of an amide group of the aliphatic nylon resin contrary to an aliphatic or aromatic ring having a ring structure which is rigid and symmetric, there is a degree of freedom to a great extent in phase transfer from a crystalline state to an amorphous state or vise versa; on this occasion, it is considered that the polyamide resin contained in the alcoholic primer is easy to function as the wedge whereupon a sufficient strength of integration can be obtained.
A fourth aspect of the present invention is characterized in that the already-molded article according to any one of the first to third aspects of the present invention is a molded article in a shape of an elongated pipe while the additional molding part according to any one of the first to third aspects of the present invention is addition-molded such that it covers an end part, in a longitudinal direction, of the already-molded article.
A resin molded article in a shape of the elongated pipe requires, regardless of a simple shape thereof, a large-sized molding die thereby allowing a blanking operation of an inside peripheral part to be difficult to be performed whereupon the shape is inappropriate for injection molding but is appropriate for continuous molding such as extrusion molding, blow molding and the like. According to the fourth aspect of the present invention, since it is possible to hold one end part of the resin molded article in a shape of the elongated pipe and, further, by performing injection molding by using the molding die having a cavity in which the additional molding part is molded, the resin molded article in a shape of the elongated pipe which is the already-molded article and the additional molding part which is molded onto an end part, in the longitudinal direction, of the resin molded article can be integrated therebetween whereupon molding can effectively be conducted without using a large-sized molding die.